CN112030270A

CN112030270A - Process for preparing refractory carbon fibers

Info

Publication number: CN112030270A
Application number: CN202010936842.3A
Authority: CN
Inventors: 赵利可; 甄崇礼; 刘峰; 李楠; 周晓兵; 刘岩
Original assignee: Beijing Sequoia Sempervirens Technology Development Co ltd
Current assignee: Beijing Sequoia Sempervirens Technology Development Co ltd
Priority date: 2020-09-08
Filing date: 2020-09-08
Publication date: 2020-12-04

Abstract

The invention discloses a process for preparing fire-resistant carbon fibers, which comprises the steps of preparing a solution of polyacrylonitrile polymers; spinning the solution of polyacrylonitrile based polymer to form filaments, injecting the filaments into a coagulation bath to coagulate the filaments, and then washing, drawing, oiling, drying and compacting the coagulated filaments; stretching in air at 0.1-5% elongation to improve the strength properties of the carbon fiber to convert it into a pre-refractory fiber; stretching the pre-fire-resistant fiber in air to form fire-resistant fiber with the elongation of 0-5%; the refractory fiber is pre-carbonized at a temperature of 300-800 ℃ in an inert atmosphere, and then stretched and carbonized at a temperature of 1000-3000 ℃ in an inert atmosphere.

Description

Process for preparing refractory carbon fibers

Technical Field

The present invention relates to a process for producing a refractory carbon fiber and a precursor fiber for the refractory carbon fiber.

Background

Carbon fibers (carbon fibers) are fibrous carbon materials, and carbon elements in their chemical composition account for 90% or more of the total mass. The carbon fiber and the composite material thereof have a series of excellent performances of high strength, high specific modulus, high temperature resistance, corrosion resistance, fatigue resistance, creep resistance, electric conduction, heat transfer, small thermal expansion coefficient and the like, and can be used as a structural material for bearing load and a functional material for playing a role. Therefore, carbon fibers and their composites have developed very rapidly in recent years.

The charring yield of the polyacrylonitrile-based carbon fiber is higher than that of the viscose fiber and can reach more than 45 percent, and the polyacrylonitrile-based carbon fiber is the carbon fiber with the widest application field and the largest yield at present because the production process, the solvent recovery, the three-waste treatment and other aspects are simpler than that of the viscose fiber, the cost is low, the raw material sources are rich, and the mechanical properties, particularly the tensile strength, the tensile modulus and the like of the polyacrylonitrile are the first of three carbon fibers.

The polyacrylonitrile protofilament is converted into a heat-resistant ladder-shaped structure after being pre-oxidized, and is converted into the carbon fiber with the disordered-layer graphite structure through low-temperature carbonization (300-. In the mechanism conversion process, the smaller ladder-shaped structural units are further subjected to crosslinking and polycondensation, and along with pyrolysis, a plurality of small molecule byproducts are released while being converted to the turbostratic graphite structure. Meanwhile, the non-carbon element O, N, H is gradually eliminated, C is gradually enriched, and finally carbon fiber with carbon content of more than 90% is formed.

Disclosure of Invention

The technical problem to be solved by the present invention is to provide a method for preparing a refractory carbon fiber in which a precursor fiber is additionally stretched or shrunk optionally in a refractory process and a carbonization process to thereby prepare a high-performance refractory carbon fiber, and to provide a precursor fiber for preparing a refractory carbon fiber.

In order to solve the technical problem, the invention discloses a process for preparing fire-resistant carbon fibers, which comprises the steps of preparing a solution of polyacrylonitrile polymers; spinning the solution of polyacrylonitrile based polymer to form filaments, injecting the filaments into a coagulation bath to coagulate the filaments, and then washing, drawing, oiling, drying and compacting the coagulated filaments; stretching in air at 0.1-5% elongation to improve the strength properties of the carbon fiber to convert it into a pre-refractory fiber; stretching the pre-fire-resistant fiber in air to form fire-resistant fiber with the elongation of 0-5%; the refractory fiber is pre-carbonized at a temperature of 300-800 ℃ in an inert atmosphere, and then stretched and carbonized at a temperature of 1000-3000 ℃ in an inert atmosphere. Specifically, the method comprises the following steps:

a process for preparing refractory carbon fibers, comprising the steps of: preparing a solution of polyacrylonitrile polymers; spinning the solution of polyacrylonitrile based polymer to form filaments, injecting the filaments into a coagulation bath to coagulate the filaments, and then washing, drawing, oiling, drying and compacting the coagulated filaments; stretching in air at 0.1-5% elongation to improve the strength properties of the carbon fiber to convert it into a pre-refractory fiber; stretching the pre-fire-resistant fiber in air to form fire-resistant fiber with the elongation of 0-5%; the refractory fiber is pre-carbonized at a temperature of 300-800 ℃ in an inert atmosphere, and then stretched and carbonized at a temperature of 1000-3000 ℃ in an inert atmosphere.

In a preferred embodiment of the present invention, in the step of carbonizing the refractory fiber, the refractory fiber is drawn at an elongation of 0 to 5.0%.

In a preferred embodiment of the present invention, in the step of carbonizing the refractory fiber, the refractory fiber is drawn at an elongation of 3.1 to 5.0%.

As a preferable aspect of the present invention, stretching is performed after the step of preparing the precursor fiber of the refractory carbon fiber so that the total elongation of the carbon fiber with respect to the precursor fiber is 5.1 to 10.0%.

In a preferred embodiment of the present invention, the matrix fiber is polyacrylonitrile fiber and has a water content of 20.0 to 50.0%.

After the technical scheme is adopted, the invention has the beneficial effects that: according to the method for preparing the refractory carbon fiber, the produced refractory carbon fiber has high axial strength and modulus, low density, high specific performance, no creep, ultrahigh temperature resistance in a non-oxidation environment, good fatigue resistance, specific heat and conductivity between nonmetal and metal, small thermal expansion coefficient, anisotropy, good corrosion resistance and good X-ray permeability. Good electric and heat conducting performance, good electromagnetic shielding performance and the like. Compared with the traditional glass fiber, the Young modulus of the refractory carbon fiber is more than 3 times of that of the refractory carbon fiber; compared with Kevlar fiber, the Young's modulus is about 2 times of that of Kevlar fiber, and the Kevlar fiber is insoluble and does not swell in organic solvents, acids and alkalis and has outstanding corrosion resistance.

Detailed Description

The following examples illustrate the invention in detail. The raw materials and various devices used in the invention are conventional commercially available products, and can be directly obtained by market purchase.

In the following description of embodiments, for purposes of explanation and not limitation, specific details are set forth, such as particular system structures, techniques, etc. in order to provide a thorough understanding of the embodiments of the present application. It will be apparent, however, to one skilled in the art that the present application may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known systems, devices, circuits, and methods are omitted so as not to obscure the description of the present application with unnecessary detail.

It will be understood that the terms "comprises" and/or "comprising," when used in this specification and the appended claims, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

It should also be understood that the term "and/or" as used in this specification and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.

As used in this specification and the appended claims, the term "if" may be interpreted contextually as "when", "upon" or "in response to" determining "or" in response to detecting ". Similarly, the phrase "if it is determined" or "if a [ described condition or event ] is detected" may be interpreted contextually to mean "upon determining" or "in response to determining" or "upon detecting [ described condition or event ]" or "in response to detecting [ described condition or event ]".

Furthermore, in the description of the present application and the appended claims, the terms "first," "second," "third," and the like are used for distinguishing between descriptions and not necessarily for describing or implying relative importance.

Reference throughout this specification to "one embodiment" or "some embodiments," or the like, means that a particular feature, structure, or characteristic described in connection with the embodiment is included in one or more embodiments of the present application. Thus, appearances of the phrases "in one embodiment," "in some embodiments," "in other embodiments," or the like, in various places throughout this specification are not necessarily all referring to the same embodiment, but rather "one or more but not all embodiments" unless specifically stated otherwise. The terms "comprising," "including," "having," and variations thereof mean "including, but not limited to," unless expressly specified otherwise.

The matrix fiber of the flame-resistant carbon fiber is a polymer containing acrylonitrile as a main component. Specifically, it is a polymer comprising acrylonitrile in an amount of 85 mol% or more than 85 mol% based on the total amount of monomers.

The monomer may include a monomer copolymerizable with acrylonitrile and acrylonitrile. The monomer copolymerizable with acrylonitrile plays a role in promoting flame resistance, and examples thereof may include acrylic acid, methacrylic acid, itaconic acid, and the like.

The polyacrylonitrile-based polymer can be prepared by solution-polymerizing a solution containing Acrylonitrile (AN) monomer using a polymerization initiator. In addition to solution polymerization, the polyacrylonitrile-based polymer may generally use ammonia as the polymerization terminator, but the present invention is not limited thereto.

A monomer containing acrylonitrile as a main component is polymerized to obtain a polymer, and then the obtained polymer is neutralized using a polymerization terminator to obtain a solution containing a polyacrylonitrile-based polymer in the form of a salt combined with ammonium ions. It may also be prepared by suspension polymerization or emulsion polymerization. Meanwhile, the polymerization initiator used in the polymerization of the monomer is not particularly limited. Preferably, as the polymerization initiator, oil-soluble azo compounds, water-soluble azo compounds, peroxides, and the like can be used. Among these compounds, water-soluble azo compounds can be preferably used in terms of safety, handleability, and industrial polymerization efficiency, and when the water-soluble azo compounds are decomposed, they do not cause oxygen generation that inhibits the polymerization reaction, and further, in the case of solution polymerization, oil-soluble azo compounds can be preferably used in terms of solubility. Specific examples of the polymerization initiator may include 2,2 '-azobis (4-methoxy-2, 4-dimethylvaleronitrile), 2' -azobis (2,4 '-dimethylvaleronitrile), 2' -azobisisobutyronitrile, and the like.

Generally, after the polymerization of the monomer, a neutralization step is subsequently performed using a polymerization terminator. The neutralization step using a polymerization terminator is intended to prevent the spinning solution containing the resulting polyacrylonitrile-based polymer from rapidly solidifying at the time of spinning the solution.

The polymer content of the solution containing the polyacrylonitrile-based polymer may be 10 to 25 wt%.

The polymerization temperature varies depending on the kind and amount of the polymerization initiator, but is preferably from 30 ℃ to 90 ℃.

The resulting solution containing polyacrylonitrile-based polymer can be used as a spinning solution for a precursor fiber for producing a flame-resistant carbon fiber. The precursor fiber of the refractory carbon fiber can be obtained by spinning the spinning solution. The spinning solution may contain an organic or inorganic solvent and the polyacrylonitrile-based polymer. Examples of the organic solvent may include dimethyl sulfoxide, dimethylformamide, dimethylacetamide, and the like.

When the above solution is used as a spinning solution for preparing a precursor fiber of a carbon fiber, there are advantages in that the solvent can be easily removed during spinning, tar or impurities can be prevented from being generated during a fire-resistant process, and the density of the filament can be maintained uniform.

The spinning method may be a dry spinning method, a wet spinning method or a dry-wet spinning method.

In the dry-wet spinning method, the spinning solution is discharged in the air (air space), surface-crystallized, and then solidified in a coagulation bath, so that a rapid increase in the drawing rate can be substantially compensated by the solution discharged in the air space, whereby high-speed spinning can be performed.

Preferably, the spinning solution is discharged through the spinneret using a wet spinning method or a dry-wet spinning method, and the discharged spinning solution is introduced into a coagulation bath to coagulate the fiber.

The setting rate or the drawing method may be appropriately determined depending on the use of the refractory fiber or the carbon fiber. The coagulation bath may be injected with a coagulation accelerator in addition to a solvent such as dimethyl sulfoxide, dimethylformamide or dimethylacetamide. As the coagulation accelerator, a solvent that does not dissolve the polyacrylonitrile-based polymer and is used in the spinning solution may be used. An example of the set accelerator may be water. The temperature of the coagulation bath and the amount of the coagulation accelerator may be appropriately determined depending on the use of the refractory fiber or carbon fiber.

The single fiber fineness of the parent fiber of the carbon fiber obtained in this manner may be 0.01 to 3.0dtex, preferably 0.05 to 1.8dtex, more preferably 0.8 to 1.5 dtex. When the single fiber fineness of the precursor fiber is too small, the carbon fiber yarn may be broken by contact with a roller or a guide (guide), so that the process of manufacturing the yarn and the process of calcining the carbon fiber may not be precisely repeated in the same manner. In addition, when the single fiber fineness of the precursor fiber is too large, the structural difference between the inner layer and the outer layer of each single fiber increases after fire resistance, a subsequent carbonization process may not be easily performed, and the tensile strength and tensile elastic modulus of the resulting carbon fiber decrease. That is, when the single fiber fineness of the matrix fiber deviates from the above range, the plastic efficiency of the carbon fiber may be rapidly deteriorated.

In the present invention, the term "single fiber fineness (dtex)" is defined as the weight (g) per 10000m of single fiber.

In particular, it is preferred that the water content of the parent fiber of the carbon fiber according to the present invention is 20 to 50%. The water content of the parent fiber of the carbon fiber may be controlled using any one of the steps of injecting the spun polyacrylonitrile-based polymer solution into a coagulation bath to coagulate the filament, and then washing, drawing, oiling, drying, and compacting (heat treating) the coagulated filament. Preferably, the water content of the precursor fiber of the carbon fiber may be controlled by controlling the heat treatment temperature in the drying and heat treatment process after the final crystal orientation of the precursor fiber reaches 85% or more than 85%, or by controlling the concentration and amount of the oil solution for improving the processability of the carbon fiber precursor in the process of carbonizing the carbon fiber precursor. Generally, the moisture content of the carbon fiber precursor may be maintained at about 4% by process moisture content level. In this case, the strength and elongation of the carbon fiber precursor can be improved by drying and compacting the carbon fiber precursor during the process and then finally stretching and drying the carbon fiber precursor.

The single fiber fineness of the parent fiber of the carbon fiber obtained in this manner may be 0.01 to 3.0dtex, preferably 0.05 to 1.8dtex, and more preferably 0.8 to 1.5 dtex. When the single fiber fineness of the precursor fiber is too small, the carbon fiber yarn may be broken by contact with a roller or a guide (guide), so that the process of manufacturing the yarn and the process of calcining the carbon fiber may not be precisely repeated in the same manner. In addition, when the single fiber fineness of the precursor fiber is too large, the structural difference between the inner layer and the outer layer of each single fiber increases after fire resistance, a subsequent carbonization process may not be easily performed, and the tensile strength and tensile elastic modulus of the resulting carbon fiber decrease. That is, when the single fiber fineness of the matrix fiber deviates from the above range, the plastic efficiency of the carbon fiber may be rapidly deteriorated. The crystal orientation of the parent fiber of the carbon fiber according to the present invention may be 85% or more than 85%, preferably 90% or more than 90%. When the crystal orientation of the parent fiber of the carbon fiber is less than 85%, the strength of the resulting parent fiber may become low. However, the present invention is based on the fact that: the mechanical properties of the carbon fiber are improved more effectively by improving the elongation and relaxation properties during carbonization than by improving the physical properties of the carbon fiber matrix. Therefore, when preparing the carbon fiber precursor, the carbon fiber precursor may be rapidly heat-treated at a temperature of 100-. Due to the above process characteristics, when the moisture content of the carbon fiber precursor is less than 20%, the moisture content thereof can be improved by adding a low-concentration oil solution to the carbon fiber precursor after final drying.

When the moisture content of the precursor fiber of the carbon fiber is controlled to 20 to 50%, the stretchability and shrinkability of the precursor fiber can be improved in the refractory and carbonization processes. In addition, in order to greatly increase the strength of the carbon fiber by improving the mechanical properties of the carbon fiber, it is preferable to improve the stretchability of the matrix fiber. Generally, a precursor fiber of the carbon fiber is prepared, and then subjected to a flame-resistant treatment, and may be subjected to a stretching treatment. When the obtained moisture content of the precursor fiber is about 4%, the elongation of the finally obtained carbon fiber is at most-10-5%, which is low. In addition, the stretching treatment may be performed in the carbonization treatment even after the fire-resistant treatment, in which case the elongation of the carbon fiber is at most-3-3% (which is lower) based on the elongation of the matrix fiber in the previous step. Therefore, in general, the carbonization conditions of the carbon fiber matrix are preferred to take into account the improvement of the mechanical properties due to the stretching rather than the process stability due to the shrinkage. However, when the precursor fiber of the carbon fiber having a water content of 20 to 50% is used, the precursor fiber may be additionally drawn under the conditions of high temperature and high orientation because water is used as a plasticizer in the fire-resistant treatment. When the elongation is increased in the fire-resistant and carbonization treatment, finally, the mechanical properties of the carbon fiber can be improved.

Thus, according to an embodiment of the present invention, a carbon fiber precursor having a high water content is used. Preferably, a carbon fiber precursor having a water content of 20 to 50% may be used. When the water content of the carbon fiber precursor is excessively high, a difference in the degree of oxidation occurs between the surface and the inside of the carbon fiber precursor during the refractory and carbonization treatment, so that a sheath-core effect is generated or the carbon fiber precursor becomes hollow. In addition, under these conditions, a peroxidation reaction of the carbon fiber matrix occurs, which significantly reduces the strength of the carbon fiber, or makes the process difficult to perform. Therefore, it is preferable that the moisture content of the carbon fiber precursor is 50% or less than 50%.

A method for producing a carbon fiber using a carbon fiber precursor having a high water content and containing a polyacrylonitrile-based polymer in a salt form will be described below.

In a process for producing carbon fibers using a carbon fiber precursor having a high water content, the process is accompanied by a general fire-resistant treatment. However, in this case, the high-temperature heat treatment is immediately and rapidly performed at 200-300 ℃, so that the carbon fiber precursor is rapidly shrunk while weak yarns are broken in the carbon fiber precursor tow and the tension of the carbon fiber precursor becomes non-uniform in the oxidation treatment, with the result that it is difficult to control the stability of the process, and a portion of the carbon fiber precursor may be rapidly burned due to the rapid heat treatment. In particular, since the shrinkage force of the carbon fiber precursor is shown to the highest degree in the temperature range of 200-240 ℃, attention is required to the stability of the process. In view of this problem, in the present invention, pre-fire resistance can be performed. In this case, it is preferred that the temperature in the refractory is higher than the temperature in the pre-refractory.

In the present invention, the temperature in the pre-refractory treatment is determined depending on the shrinkage rate of the carbon fiber and the plasticity of moisture. Therefore, if the temperature in the pre-refractory treatment is below 180 ℃, there is a problem in that the carbon fiber precursor is not sufficiently compacted; if the temperature is higher than 220 c, there is a problem in that water is rapidly volatilized, so that the stretchability of the carbon fiber precursor is rapidly deteriorated. In addition, in the pre-refractory treatment, when the elongation of the carbon fiber precursor is more than 5%, there is a problem in that the carbon fiber precursor is excessively hardened, and thus a portion of the carbon fiber precursor is broken, thereby causing firing during the refractory process. Therefore, the maximum elongation is preferably 5% or less than 5%, and the elongation is preferably 0.1 to 5% in terms of improving strength. The carbon fiber precursor pre-refractory in this manner is then stretched at a temperature of 200-300 ℃ while being refractory.

The pre-fire-resistant treatment is performed such that the carbon fiber precursor having a high water content of 20-50% is pre-fire-resistant at a temperature range of 180-220 deg.c while being drawn at an elongation of-10-0.1% or 0.1-5%, considering that the carbon fiber precursor is shrunk to a maximum elongation of 5%. That is, since the impact caused by the shrinkage of the carbon fiber precursor before the carbon fiber precursor is introduced into the refractory furnace can be relaxed in this temperature range, both the effect of process stability and the effect of improving physical properties can be achieved.

In order to produce carbon fibers having high strength, it is preferable that the elongation of the refractory carbon fiber precursor is 0 to 5% with respect to the pre-refractory carbon fiber precursor. More preferably, the elongation is 0 to 0.1%. In this case, the elongation of the refractory carbon fiber precursor may be-5 to 5% with respect to the pre-refractory carbon fiber precursor. Here, the carbon fiber precursor having a high water content is subjected to pre-fire resistance and then fire resistance to be imparted with high strength. Therefore, the elongation of the refractory carbon fiber matrix is higher than that of a carbon fiber matrix obtained by conventional refractory.

Then, the refractory carbon fiber precursor is stretched at a temperature of 300-800 ℃ in an inert gas atmosphere according to the purpose while being pre-carbonized, and then further stretched at a high temperature of 1000-3000 ℃ in an inert gas atmosphere according to the purpose while being carbonized, thereby preparing the carbon fiber.

The pre-carbonization or carbonization of the refractory carbon fiber precursor is carried out under an inert gas atmosphere. Examples of the gas used in the inert gas atmosphere may include nitrogen, argon, xenon, and the like. The temperature in the carbonization of the refractory carbon fiber precursor may be set to 1000-3000 ℃. Generally, the tensile modulus of the resulting carbon fiber increases with increasing temperature in carbonization of the refractory carbon fiber matrix, but the tensile strength is at its highest at 1300-. Therefore, in order to increase both the tensile strength and the tensile elastic modulus of the carbon fiber, the maximum temperature in carbonization of the refractory carbon fiber matrix may be 1200-1700 ℃, preferably 1300-1500 ℃.

Meanwhile, the elongation of the carbon fiber precursor in carbonization after the oxidation stabilization may be-10.0 to 5.0%, preferably-5.0 to 5.0%, and preferably 3.1 to 5.0%. The reason why the elongation can be increased at the time of carbonization is that the carbon fiber precursor having a high water content has been subjected to a pre-refractory and refractory process. When a carbon fiber produced by making a carbon fiber precursor having a high water content pre-refractory, fire-resistant and then carbonized is stretched so that the elongation of the carbon fiber with respect to the carbon fiber precursor is-10 to 10%, preferably 5.1 to 10.0%, this is preferable in terms of improving the mechanical properties of the carbon fiber and improving the process stability.

The resulting carbon fiber may be electrolyzed to modify the surface thereof. As the electrolyte solution used in the electrolysis of carbon fibers, acidic solutions such as sulfuric acid, nitric acid, hydrochloric acid, and the like, and alkaline aqueous solutions such as sodium hydroxide, potassium hydroxide, tetraethylammonium hydroxide, ammonium carbonate, ammonium bicarbonate, and salts thereof can be used. Here, the amount of electricity used for electrolyzing the carbon fiber may be appropriately selected depending on the degree of carbonization of the carbon fiber used. In the fiber-reinforced composite material obtained by electrolysis of the carbon fibers, the adhesion between the fiber-reinforced composite material and the carbon fiber matrix can be optimized, so that the following problems can be overcome: a problem that the composite material becomes brittle due to very strong adhesion, or a problem that strength characteristics of the composite material in a non-fiber direction are not exhibited due to deterioration of adhesion between the composite material and a resin in spite of tensile strength of the composite material in a fiber direction. Therefore, in the resulting fiber-reinforced composite material, the strength characteristics thereof are exhibited unevenly in the fiber direction and the non-fiber direction. After the electrolysis of the carbon fibres, the electrolyzed carbon fibres may be sized. The sizing agent for sizing the electrolyzed carbon fiber may be appropriately selected from sizing agents suitable for the resin, depending on the kind of the resin used. The carbon fiber of the present invention is a prepreg, and can be used for manufacturing aircraft components, pressure vessel members, automobile members, and sports equipment (such as fishing rods, golf clubs, and the like) by various forming methods such as autoclave molding, resin transfer molding, and filament winding (filamentwinding), and the like.

In addition, considering that the carbon fiber is used for manufacturing an aircraft, it is important to reduce the weight of the carbon fiber, and it is preferable that the maximum temperature in carbonization of the carbon fiber matrix is 1700-. As the maximum temperature increases in carbonization of the carbon fiber matrix, the tensile elastic modulus of the carbon fiber increases, but the carbon fiber may be graphitized. Due to graphitization of the carbon fiber, the carbon surface of the carbon fiber is liable to wrinkle due to its growth and partial pressure, and as a result, the compressive strength of the carbon fiber may be reduced. Therefore, the temperature during carbonization is determined in consideration of the balance between the tensile elastic modulus and the strength resistance of the carbon fiber.

Examples

The present invention will be described below with reference to examples and tables. It is to be understood that the embodiments described herein are merely illustrative and explanatory of the invention and are not restrictive thereof.

[ Table 1]

The strength of the carbon fibers obtained in examples 1 to 6, reference example 1 and comparative example 1 was evaluated by the following method, and the results thereof are given in table 2 below.

(1) Method for evaluating strength of carbon fiber referring to Japanese unexamined patent application publication No.2003-161681, according to JISR760, physical properties of carbon fiber are evaluated as follows: a fiber strand evaluation device was prepared, carbon fibers were impregnated with epoxy resin, and then carbon fiber bundles were stretched straight. Here, the distance between the carbon fibers was 100mm, the measurement speed was 60mm/min, and the evaluation was performed 10 times.

[ Table 2]

	Fiber strand strength (strandstrength) (MPa)
		Example 1	3500
Example 2	4410
		Example 3	4600
Example 4	4730
		Example 5	3960
Example 6	4480
		Reference example 1	2900
Comparative example 1	4070

< examples 1 to 4>

In examples 1 to 4, elongation was different from each other in pre-fire resistance, fire resistance and carbonization as given in the following table 1. In this case, it is understood that the elongation in each process is based on the difference in the treatment rate before and after each process.

A polyacrylonitrile-based copolymer in the form of an ammonium salt is obtained by polymerizing 95 mol% of acrylonitrile, 3 mol% of methacrylic acid and 2 mol% of itaconic acid by a solution polymerization method using dimethylsulfoxide as a solvent and then adding ammonia thereto in the same amount as that of itaconic acid to neutralize the reaction product, thereby obtaining a spinning solution containing 22 wt% of the polyacrylonitrile-based copolymer and, in view of the use of carbon fiber for the manufacture of aircraft, it is important to reduce the weight of carbon fiber, and it is preferable that the maximum temperature in the carbonization of the carbon fiber matrix be 1700-. As the maximum temperature increases in carbonization of the carbon fiber matrix, the tensile elastic modulus of the carbon fiber increases, but the carbon fiber may be graphitized. Due to graphitization of the carbon fiber, the carbon surface of the carbon fiber is liable to wrinkle due to its growth and partial pressure, and as a result, the compressive strength of the carbon fiber may be reduced. Therefore, the temperature during carbonization is determined in consideration of the balance between the tensile elastic modulus and the compressive strength of the carbon fiber.

The above spinning solution was discharged through two spinnerets (each having a temperature of 45 c, a diameter of 0.08mm and 6000 holes) and then introduced into a coagulation bath maintained at 45 c and filled with an aqueous solution containing 40% dimethylsulfoxide, thereby preparing a coagulated yarn.

The coagulated yarn was washed with water, then drawn in hot water five times, and then a net-like modified silicone oil solution was added to the yarn to obtain an intermediate drawn yarn.

The intermediate-drawn yarn was dried using a hot roll and then drawn in pressurized steam to obtain a polyacrylonitrile-based fiber bundle having a total elongation of 10, a single fiber fineness of 1.5dtex and a filament number of 12000. The obtained polyacrylonitrile-based fiber bundle is referred to as a precursor fiber of carbon fiber.

In this case, after the intermediate drawn yarn is drawn in pressurized steam, the heat treatment temperature is controlled to 80 to 120 ℃ during the heat treatment of the drawn intermediate drawn yarn, thereby obtaining precursor fibers having different moisture contents. In this case, the moisture content may be realized by converting the amount of the spinning solution discharged from the spinneret into the fineness of the wound parent fiber and the winding speed of the parent fiber, and may be analyzed using GC-MASS (Varian4000GC-MS) as follows.

GC-MASS analysis

Instrument Varian4000GC-MS

Stationary phase VF-5ms (30 m.times.0.25 mm.times.0.25 μm)

Mobile phase He,1.0ml/min

Temperature raising program from 80 deg.C, 2min to 280 deg.C, 8min (@20C/min)

Injection of 0.4. mu.l with split 20:1,250 deg.C

Detection EI mode (28 to 500m/z scanning)

The obtained polyacrylonitrile-based fiber bundles were each subjected to flameproofing (accompanied by stretching) at a winding speed of 4m/min at 200 ℃ for 6 minutes without twisting under an air atmosphere and then flameproofed (accompanied by stretching) for 80 minutes in a 4-stage hot air oven at a temperature range of 220 ℃ and 270 ℃. Then, the above-mentioned fire-resistant polyacrylonitrile-based fiber bundle is pre-carbonized at 400-700 ℃ under an inert atmosphere to remove exhaust gas, and then finally carbonized (accompanied by drawing) at 1350 ℃ to produce carbon fibers having improved strength.

< example 5>

A carbon fiber was prepared using a precursor fiber having the same moisture content as that of the precursor fiber in example 1, except that the elongation of the precursor fiber was set to-2.5% during the fire resistance of the precursor fiber and 0.5% during the carbonization thereof.

< example 6>

A carbon fiber was prepared using a precursor fiber having the same moisture content as that of the precursor fiber in example 1, except that the elongation of the precursor fiber was set to 1.5% in the fire-resistant process of the precursor fiber.

< reference example 1>

Polyacrylonitrile-based copolymer in the form of ammonium salt was prepared by polymerizing 95 mol% acrylonitrile, 3 mol% methacrylic acid and 2 mol% itaconic acid using dimethyl sulfoxide as a solvent by a solution polymerization method and then adding ammonia thereto in the same amount as that of itaconic acid to neutralize the reaction product, thereby obtaining a spinning solution containing 22 wt% polyacrylonitrile-based copolymer.

The coagulated yarn was washed with water, then drawn four times in hot water, and then a net-like modified silicone oil solution was added to the yarn to obtain a drawn yarn. The drawn yarn was dried using a 150 ℃ hot roll and then drawn in pressurized steam to give total elongation

A polyacrylonitrile-based fiber bundle of 10, a single fiber fineness of 1.5dtex and a filament number of 12000. The polyacrylonitrile fiber bundle is heat treated at 135 deg.c in hot air drier to obtain the mother fiber of carbon fiber. The moisture content of the parent fiber of the obtained carbon fiber was measured in the same manner as in example 1 to be 4.5%. The obtained polyacrylonitrile-based fiber bundle was subjected to fire resistance (while stretching the polyacrylonitrile-based fiber bundle at an elongation of 2.5%) at a winding speed of 4m/min for 80 minutes in a 4-stage hot air oven at a temperature range of 220-270 ℃ in an air atmosphere without twisting the polyacrylonitrile-based fiber bundle.

Then, the above-mentioned fire-resistant polyacrylonitrile fiber bundle was pre-carbonized at 400-700 ℃ under an inert atmosphere, and then finally carbonized at 1350 ℃ (while the polyacrylonitrile fiber bundle was stretched at an elongation of-1.5%) to prepare a carbon fiber.

< comparative example 1>

A carbon fiber was prepared using a precursor fiber having the same water content as that of the precursor fiber in example 1, except that the fire-retardancy of the precursor fiber was performed at 220-270 ℃ for 80 minutes in an air atmosphere while the stretching of the precursor fiber was performed at an elongation of 1.5%, without performing the pre-fire-retardancy of the precursor fiber. Then, the above refractory precursor fiber was pre-carbonized at 400-700 ℃ under an inert gas atmosphere, and then finally carbonized at 1350 ℃ (while the precursor fiber was drawn at an elongation of 1.5%). In this case, there is a disadvantage in that oxidation stability and carbonization treatment of the matrix fiber of the carbon fiber are unstable in terms of processability because the matrix fiber is locally broken. In particular, there are disadvantages in that the partially broken precursor fiber deteriorates the strength of the carbon fiber, and in that the carbon fiber is broken because the partially broken precursor fiber remains as a lap (wrap) in the process.

The above-mentioned embodiments are only used for illustrating the technical solutions of the present invention, and not for limiting the same; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; such modifications and substitutions do not substantially depart from the spirit and scope of the embodiments of the present invention, and are intended to be included within the scope of the present invention.

Claims

1. A process for preparing refractory carbon fibers, comprising: the method comprises the following preparation steps: preparing a solution of polyacrylonitrile polymers; spinning the solution of polyacrylonitrile based polymer to form filaments, injecting the filaments into a coagulation bath to coagulate the filaments, and then washing, drawing, oiling, drying and compacting the coagulated filaments; stretching in air at 0.1-5% elongation to improve the strength properties of the carbon fiber to convert it into a pre-refractory fiber; stretching the pre-fire-resistant fiber in air to form fire-resistant fiber with the elongation of 0-5%; the refractory fiber is pre-carbonized at a temperature of 300-800 ℃ in an inert atmosphere, and then stretched and carbonized at a temperature of 1000-3000 ℃ in an inert atmosphere.

2. The process for preparing a refractory carbon fiber according to claim 1, wherein: in the step of carbonizing the refractory fiber, the refractory fiber is drawn at an elongation of 0 to 5.0%.

3. A process for preparing a refractory carbon fiber according to claim 2, wherein: in the step of carbonizing the refractory fiber, the refractory fiber is drawn at an elongation of 3.1 to 5.0%.

4. The process for preparing a refractory carbon fiber according to claim 1, wherein: the step of preparing a parent fiber of the refractory carbon fiber is followed by drawing so that the total elongation of the carbon fiber with respect to the parent fiber is 5.1 to 10.0%.

5. The process for preparing a refractory carbon fiber according to claim 4, wherein: the parent fiber is polyacrylonitrile fiber and has water content of 20.0-50.0%.